1. Field of the Invention
The present invention relates to an image forming apparatus capable of executing double-sided printing, a control method therefor, and a storage medium.
2. Description of the Related Art
Recently, printers have achieved high resolutions, with printers having resolutions of 600 dpi (dots/inch) and 1,200 dpi making their debuts. Those printers switch the resolution between 600 dpi and 1,200 dpi by changing the conveying speed of printing paper (sheet). More specifically, when a sheet conveying speed corresponding to 600 dpi is defined as the first constant speed, printing at a 1,200-dpi double resolution is implemented by conveying a sheet at a speed (half speed) that is half the first constant speed. Therefore, in 1,200-dpi printing, the sheet conveying speed is halved, halving the productivity of 600 dpi printing. To implement 1,200-dpi printing at the first constant speed, the rotational speed of a polygon mirror which scans a laser beam is doubled, or the number of laser beams is doubled, raising the hardware cost. To implement high-resolution printing at low cost, switching of the resolution by switching the conveying speed is an indispensable technique. For example, Japanese Patent Laid-Open Nos. 2000-181275 and 2002-23576 have proposed this technique.
As a method of forming images on the obverse and reverse surfaces of many sheets, a circulation paper conveying method is used. In this method, a sheet is sent to a transfer unit configured to transfer an image, and an image is printed on one surface of the sheet. The sheet is then sent to an inversion unit and inverted. The inversed sheet is then sent again to the transfer unit, and an image is transferred on the reverse surface, thereby printing images on the two surfaces of the sheet. To increase productivity by shortening the interval between sheets, double-sided circulation control is performed in double-sided printing. More specifically, instead of feeding and refeeding one sheet, a plurality of sheets are fed at once to successively print images on their first surfaces, and the sheets each bearing the image on the first surface are sent to a double-sided path. Then, images for the first surfaces of newly fed sheets, and images for the second surfaces of the sheets conveyed via the double-sided path are alternately printed.
As an image forming apparatus using the circulation paper conveying method, Japanese Patent No. 03768785 has proposed an image forming apparatus which decides a double-sided circulating sheet count based on the paper size and paper type of a sheet to be fed. Even if there is a sheet to be printed subsequently, control to temporarily interrupt double-sided circulation is performed under a given condition. For example, when the paper size switches from A4 to A3, double-sided circulation is interrupted. This is because the circulating sheet count switches depending on the paper size and paper type. Also, when the paper type switches (from plain paper to thick paper), double-sided circulation is interrupted because the conveying speed switches.
Conventional sheet double-sided circulation control in Japanese Patent No. 03768785 is executed for each sheet. This is because the double-sided circulating sheet count is obtained based on sheet information such as “paper size” and “paper type”. The sheet conveying speed is also switched based on “paper type”. This is because the conveying speed is decreased for a sheet having a large grammage such as thick paper, so as to increase the heat capacity and implement stable fixing.
However, in Japanese Patent No. 03768785, whether to interrupt double-sided circulation is determined not based on the resolution but based on sheet information such as “paper size” and “paper type”. For this reason, even if the conveying speed changes between printing sheets when printing images having different resolutions on a plurality of printing sheets, double-sided circulation cannot be properly interrupted, generating a jam.